A witness tells 10 News he saw the helicopter's main two-bladed rotor literally spin off of the aircraft, at which point it immediately plummeted straight down into the water below.

It's not known at this time what caused the crash.

Detective Larry McKinnon with the Hillsborough County Sheriff's Office says one person is believed to have been in the helicopter when it crashed. McKinnon adds they are currently treating it as a search and rescue operation.

911 reportedly received two calls: one person reported hearing an explosion and another person saw the chopper go down.

At least 15 Tampa Fire units, along with additional Hillsborough Fire Rescue units, are responding to the scene.